One problem which has arisen in the operation of fuel cell stacks to produce electricity is the maintenance of optimal operating temperatures at the end cells in the stack. This is particularly applicable in start-up of the stack wherein the individual cells must be brought up to proper operating temperatures by circulation of preheated coolant through the stack. In the stacks in question, each end will comprise a thermal and electrical insulation plate outwardly adjacent to the stack of cells and a metal pressure plate outwardly adjacent to the insulation plate. The insulation plate is intended to retard heat transfer out of the stack from the endmost cells in the stack. It has not, however, proven to be entirely satisfactory, and provisions have been made in the prior art to enhance the desired heat retention at the ends of the stack. One approach which has been made to increase the heat retention capabilities of the ends of the stack is to position a pair of coolant plates adjacent to each other outwardly of the endmost cells at each end of the stack and inwardly of the insulation plate. This has proven to be adequate during start-up to bring the end cells quickly up to operating temperature, but it has proven to be unsatisfactory during normal operation as the end cells tend to operate at lower than optimum temperature because of increased cooling. Another approach involves the placement of an electric heater on the inside surface of each of the insulation plates directly adjacent to the end cells in the stack. The heater is turned on during start-up and then turned off when the stack reaches proper operational temperatures. During normal operation, the heater cycles on and off to maintain satisfactory temperature. The heater is recessed directly into the insulation plate. This solution is operationally satisfactory, but it requires use of electricity which is undesirable. More importantly, it also exposes the end cells to damaging operating conditions if these heaters fail in either the on or off mode.
The system of this invention solves the problem by positioning coolant circulating plates at each end of the stack between the insulating plates and the end pressure plates. These auxiliary coolant circulating plates are connected into the main stack coolant circulating loop so that they circulate the same coolant at the same temperatures as is circulating through the rest of the stack. The net effect of this system is that there is little or no heat lost from the end cells in the stack through the insulating plate. This is because both the inside and outside of the insulating plate are kept at substantially the same temperature when coolant is being circulated through the system. The only heat lost is through the pressure plate, and that has minimal effect on the end cells in the stack. When the stack is started up, preheated coolant is circulated through all of the cooling plates in the stack to bring the cells up to operating temperature. During full operation, both sides of the insulating plate will be at the optimum operating temperature and the end cells will not overheat. The solution does not involve any additional input of energy, utilizes standard cell components, and does not complicate the stack configuration at all.
It is therefore an object of this invention to provide a fuel cell stack having improved cell temperature controls on the end cells while eliminating all failure modes associated with automatic electrical heaters, i.e., sensors, logic, relays, wiring.
It is a further object of this invention to provide a fuel cell stack of the character described which minimizes heat loss from the end cells in the stack.
It is an additional object of this invention to provide a fuel cell stack of the character described wherein both the inside and outside surfaces of end insulating plates in the stack are kept at substantially the same operating temperatures under all cell operating conditions so as to minimize heat transfer from the end cells in the stack outwardly across the insulating plates.
It is another object of this invention to provide a fuel cell stack of the character described wherein the end insulating plates in the stack are sandwiched between coolant circulating plates ducted to a common coolant circulating system.
These and other objects and advantages of this invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the accompany drawings.